An electronic device such as a personal computer (PC) may enlarge the performance and the function by adding an extension unit depending on a user's purpose and a usage.
The electrode terminal of the extension unit is inserted into an extension slot formed in a motherboard mounted on the electronic device such as PC. A variety of the extension units are available depending on usages. A variety of fixing positions and fixing methods exist depending on a kind of an enlargement slot to which the extension unit is connected, and on a structure of housing.